Crystalline zeolitic molecular sieves prepared by hydrothermal crystallization from reaction mixtures containing organic templating agents can, in general, be prepared in forms more highly siliceous than those which are synthesized in the absence of the organic reagents. It has been proposed that the crystallization mechanisms are different. In the case of the low-silica species, the mechanism involves the formation of stabilized metal cation aluminosilicate complexes and is controlled largely by the aluminate and aluminosilicate solution chemistry. In the case of the highly siliceous molecular sieves, a true templating or clathration mechanism is involved in which the organic reagent, typically an alkylammonium cation, forms complexes with silica via hydrogen bonding interactions. These complexes template or cause replication of the structure via stereo-specific hydrogen bonding interaction of the quaternary ammonium cation with the framework oxygens. Whatever the synthesis mechanism, the templated crystal structures in many instances can be directly synthesized over a very wide range of silica alumina (SiO.sub.2 /Al.sub.2 O.sub.3) ratios. At the extreme upper end of the range, the compositions are essentially silica polymorphs containing no AlO.sub.2 tetrahedra in their framework structure. These highly siliceous molecular sieves, particularly those having SiO.sub.2 /Al.sub.2 O.sub.3 molar ratios of 200 or greater, are highly hydrophobic and strongly organophilic. As such, they have found extensive use in molecular sieve separations involving organic substrates, particularly those in which water vapor cannot be entirely excluded from contacting the adsorbent.
The separation of normal C.sub.4 hydrocarbons from isobutylene, with silicalite was disclosed in Neuzil et al U.S. Pat. No. 4,455,445. The patentees stated that undesired side effects of olefin dimerization and polymerization could be substantially completely eliminated. However, with the need for more and more highly purified olefinic monomers for production of advanced polymers, the standards for permissible catalytic polymerization side effects have been lowered. Producers of polymers, detergents, etc., are demanding monomers having virtually no reactivity as determined by the "Parr Bomb" test, infra.
It is found, however, that despite the low concentration of acidic sites in highly siliceous molecular sieves due to the low concentration of AlO.sub.2 tetrahedral units in the structure, the relatively few such sites which are present posses a very considerable acid strength and the molecular sieve can exhibit a very significant catalytic activity. Moreover, even zeolitic or silica molecular sieves which have SiO.sub.2 /Al.sub.2 O.sub.3 ratios greatly in excess of 200 are reactive because of the ubiquitous presence of aluminum impurities in the silica and other reagents employed in the synthesis. Even if only a fraction of this impurity aluminum is incorporated into the crystal lattice, the remaining aluminum-containing compounds occluded in the pore system of the crystal structure are believed to exhibit at least some catalytic activity. Accordingly, in adsorptive separation processes using these molecular sieves, it is frequently found that the adsorbed constituents of the mixture being separated are, to an undesirable degree, converted to objectionable species which reduce the purity of the separated products.
Several previous attempts to reduce the acid-catalytic activity of silicalite have been reported. In Flanigen et al U.S. Pat. No. 4,073,865, the F-silicalite polymorph reported there has a reduced tendency to catalyze reactions of the feed in the presence of surface hydroxyl groups of adsorbed water. However, F-silicalite is very difficult and expensive to make.
In Canadian Patent 2,004,430, certain zeolite compositions were disclosed for use in the separation of n-olefins and n-paraffins from branched-chain olefins and paraffins, aromatic hydrocarbons and sulfur-containing compounds. The adsorbent used in the separation process of the patent is exemplified by silicalite which is first acid-treated and then base-treated. Canadian Patent 2,003,740 contains a similar disclosure. Both patents disclose several methods of preparation of the treated adsorbents, e.g., silicalite or ZSM-5 first treated with an acid and then treated with NaOH or anhydrous ammonia.
The separations referred to herein can be practiced in fixed or moving adsorbent bed systems or a countercurrent simulated moving bed system, such as described in Broughton U.S. Pat. No. 2,985,589, incorporated herein by reference and Neuzil et al, supra. Equipment utilizing these principles are familiar, in sizes ranging from pilot plant scale (deRosset U.S. Pat. No. 3,706,812) to commercial scale in flow rates from a few cc per hour to many thousands of gallons per hour.